UVIS Calibration Update Greg Holsclaw Bill McClintock June 27, 2011

Outline Recent stellar calibrations Sensitivity decline and Spica variability Alternative origin of the FUV flat-field, revisited

Recent stellar calibrations EUV FUV These plots show the total signal on the detector as a function of star position along the slit

All stellar calibrations EUV FUV These plots show the total signal on the detector as a function of star position along the slit

Decline in FUV in sensitivity over time Total signal from Spica vs row position of the image, for all calibration observations. Mean value of the signal when the star was located between rows 18 and 22, then normalized to the first.

Data vs model Total FUV signal with linear trend divided out. Also shown is the predicted variation in flux from the model. Data vs model Variation in flux is given by [Shobbrook, 1969; Sterken et al, 1986]: dE = A M2/M1 (R/D)3 (1+e cos(TA+Φ))3 (1-3cos2(TA+TA0+Φ) sin2i )

Background on Alpha Vir (Spica) Spica is a non-eclipsing double-lined spectroscopic binary system Though not spatially resolvable, each component is detectable through measurements of out-of-phase Doppler shifts in the constituent spectral lines Non-eclipsing due to large apparent orbital inclination of ~70 degrees Both stars are of a similar spectral class: Primary: B1V Secondary: B4V Spica is the brightest rotating ellipsoidal variable star The stars have a distorted ellipsoidal shape due to mutual gravitation effects As the components revolve, the visible area (and thus the observed flux) changes with orbital phase Since this is a geometric effect, it should be roughly wavelength-independent Orbital period is 4.01454 days Amplitude of flux variation in V-filter ~3% The primary of Spica is a Cepheid variable Periodic variation in the pulsating primary star is much shorter than the system’s orbital period and about a factor of 2 less in magnitude Period is 4.17 hours Amplitude of flux variation in V-filter ~1.5% This short-term variation, identified in 1968, became undetectable in the early 1970’s (but may return again due to precession of the primary’s rotation axis relative to the orbital plane, which has a period of 200 years [Balona, 1986]) http://observatory.sfasu.edu

Ellipsoidal variation model Variation in flux is given by [Shobbrook, 1969; Sterken et al, 1986]: dE = A M2/M1 (R/D)3 (1+e cos(TA+Φ))3 (1-3cos2(TA+TA0+Φ) sin2i ) Where: A=0.822 (wavelength dependent “photometric distortion”) M2/M1 = 1/1.59 (ratio of masses) R = 7.6 Rsun = 5.2858e6 km (polar radius of primary) D = 1.92916e7 km (mean separation between stars) e = 0.14 (orbital eccentricity) TA (true anomaly) T0 = 4.01454 days (orbital period) TA0 = 150 degrees (apparent angle to line of apsides in year 2005, has precession period of 128 years) i = 65.9 degrees (orbital inclination) Φ = empirical phase shift, a free parameter to match with data One period of the expected variation in flux from Spica

Rethinking the FUV flat-field

Extended source vs point source An extended source appears to exhibit flat field effects The total signal from a point source, as a function of position on the detector, does not Why?

Local mislocation of counts? Hypothesis: photoevents that occur within the geometric area of an adjacent spatial pixel are erroneously counted Say a photoevent located in this pixel is counted by the pixel above

Local mislocation of counts? The effect of these mislocations is a change in the effective width and position of spatial pixels. The flat-field variation is caused not by changes in QE, but by changes in effective area. 80μm 100μm 140μm 60μm 120μm

Sensitivity Variations in sensitivity from row-to-row can be separated into: Effective area Quantum efficiency (e.g. “burn” effects) Sensitivity [counts/s per radiance unit] Radiance [ph/s/cm^2/ster/nm] Irradiance [ph/s/cm^2/nm]

Consequences Images constitute an irregular grid of nonuniform pixel size More accurate measure of unresolved targets: full-disk reflectance of icy satellites stars Similar behavior in the spectral dimension?

Evidence Row width is correlated with the row-to-row variation from an extended object Row position is uneven in the FUV, more even in EUV Row width is unaffected by the starburn

Spatial profile for a single pixel This shows the detected counts for a single pixel as the star image is slewed along the detector We can measure the time the star crossed the center of the pixel and the width of the profile

Row width vs spectral column Unclear why the apparent width increases toward shorter wavelengths

Average row width Average of columns 600-1000 Colors indicate different observations spanning 2005 to 2011 Insensitive to changes in response at the starburned rows

Correlation of flat-field with row width Fractional change in row width does not match the variation in the flat-field

Pixel center position We can find the time at which the image crosses each pixel Deviations from a line indicates either: Spacecraft slew is not smooth Pixel position is not even

FUV rows appear unevenly distributed A strong correlation exists from column-to-column in the apparent position of FUV pixels That is, entire rows systematically deviate from the expected position

Future plans The response of each row is undersampled because the image moves by ~90microns during an integration period (currently 45 sec) This results in a poor determination of pixel location and width Therefore, we would like to plan a stellar calibration with either a shorter integration time (10 or 20 sec) or a slower spacecraft slew rate The data volume would be 2-3x larger and the time commitment 2-3x longer than the current observation

Ratio of UVIS to SOLSTICE This plot shows the ratio of UVIS spectra (1.1nm bins) to SOLSTICE over 130-180 nm Some stars are located in starburned rows WITH flat-field

α Vir / Spica Measurements vs Model As calculated, the Kurucz model exceeds the measured irradiance spectra by ~20% This discrepancy is likely due to uncertainties in the model parameters (distance, radii, or temperature) Therefore, the Kurucz spectrum will be visually adjusted by a factor of 0.8 to match the measurements in the FUV

α Vir / Spica Measurements vs Model In the EUV, the Kurucz model is in rough agreement with the EUVE measurement However, the UVIS and Rocket measurements are also in rough agreement with each other, but significantly lower than Kurucz and EUVE

To do Modify the current FUV calibration to better agree with SOLSTICE Absolute calibration updates after every star calibration

Observation planning?